(1) Field of the Invention
The present invention relates to a process for removing a coating from a substrate made from a nickel-base superalloy and to a process for treating a nickel-base superalloy.
(2) Prior Art
Cast nickel-base superalloys used in turbine engine components can be coated with MCrAlY type overlay coatings that typically contain about 8-12% aluminum. These coatings extend the life of the components that they are applied to. Some nickel-base superalloys contain a high volume fraction of γ/γ′ eutectic phase which is highly enriched in aluminum and of relatively large scale (up to about 5-10 mils in diameter) compared to the surrounding microstructure. Solution heat treatment of such alloys does not fully eliminate these phases.
During manufacturing rework or aftermarket repair, coatings are removed using mineral acids. 70-100 v/o hydrochloric acid is typically used to remove MCrAlY type coatings which preferentially leaches the aluminum in coatings containing relatively elevated levels of aluminum, but does not attack the base alloy which contains much lower levels of aluminum. On a significant number of coating and base alloy systems, the mineral acids used preferentially attack the coating without significant chemical attack or corrosion of the base alloys. The result is that the coating is removed without damaging the part. However, alloys having high volume fractions of γ/γ′ eutectic phase have exhibited more base alloy pitting type attack than similar alloys with low volume fractions of the γ/γ′ eutectic phase. This is in part due to the hydrochloric acid selectively attacking the large surface connected aluminum enriched eutectic phases. Consequently, a need exists for a coating strip process to remove MCrAlY type coatings from these alloys having high volume fractions of γ/γ′ eutectic phase with minimal base ally attack.